Why do you have butterflies when you kiss your boyfriend?

This may not happen for all people but for some it could be because it is the very first time you're kissing your boyfriend (just starting this relationship) or it could happen almost everytime because you don't want to 'stuff up' and want it to be perfect.

And also one of you could go in for a more passionate kiss rather than just a small peck or a genuine kiss and you don't know exactly what you should do.